This invention relates generally to flow control systems for controlling flows of fluids. More particularly, this invention relates to flow control systems for controlling flows of returning drilling fluids for kick prevention during the drilling of petroleum producing wells, such as offshore wells for hydrocarbons.
The exploration and production of hydrocarbons from subsurface formations have been done for decades. Due to the limited productivity of aging land-based production wells, there is growing interest in hydrocarbon recovery from new subsea wells.
Generally, for drilling an offshore well, a rotatable drill bit attached to a drill string is used to create the well bore below the seabed. The drill string allows control of the drill bit from a surface location, typically from an offshore platform or drill ship. Typically, a riser is also deployed to connect the platform at the surface to the wellhead on the seabed. The drill string passes through the riser so as to guide the drill bit to the wellhead.
During well drilling, the drill bit is rotated while the drill string conveys the necessary power from the surface platform. Meanwhile, a drilling fluid is circulated from a fluid tank on the surface platform through the drill string to the drill bit, and is returned to the fluid tank through an annular space between the drill string and a casing of the riser. The drilling fluid maintains a hydrostatic pressure to counter-balance the pressure of fluids coming from the well and cools the drill bit during operation. In addition, the drilling fluid mixes with material excavated during creation of the well bore and carries this material to the surface for disposal.
Under certain circumstances, the pressure of fluids entering the well from the formation may be higher than the pressure of the drilling fluid. This may cause the flow of the returning drilling fluid to be significantly greater than the flow of the drilling fluid in the drill string being presented to the well. Under exceptional circumstances, there is potential for catastrophic equipment failure and the attendant potential harm to well operators and the environment.
Well operators are keenly aware of the destructive potential of such unwanted influxes and continuously monitor drilling fluid inflows and outflows at the surface in order to detect surface changes in well flows. For example, the drilling fluid level in the fluid tank on the surface platform is monitored during circulation of the drilling fluid to determine if flow changes within the well are occurring. However, such methods may be imprecise and need a relatively longer time to detect and respond to a flow change within the well.
When an influx is detected, operators need to increase the hydrostatic pressure of the drilling fluid by shutting the well in with rams or annulars in a blow-out preventer that are intended for this purpose and then replacing the drilling fluid with fluid of higher density. This operation may take on the order of half a day and represent a significant impact on drilling productivity.
Therefore, there is a need for new and improved flow control systems for which may be used to detect pressure changes occurring during the creation of hydrocarbon production wells, and to control the flow of returning drilling fluids to surface platforms efficiently, for example offshore oil drilling platforms.